1. Field of the Invention
The invention relates to storable sealant and adhesive compositions showing extended shelf life in the presence of oxygen but being rapidly curable upon exclusion of oxygen. Such compositions are generally referred to as anaerobically curable. They comprise polymerizable acrylate or methacrylate ester monomers and a redox system as polymerization initiator and accelerator.
2. Description of the Prior Art
It is generally known to add stabilizers to materials containing vinyl compounds in order to prevent premature polymerization during storage and as a result of adding said stabilizers an adequate storage stability is normally obtained. Generally, no particular difficulties are encountered in stabilizing radically polymerizable monomers at ambient temperature. However, considerable problems arise if the monomers already contain an accelerator system formed by a peroxidic initiator and an amine, as is the case with anaerobically curable adhesives. The particular difficulty is that in most cases effective stabilizers reduce the curing rate of such adhesive mixtures.
A large number of substances has been proposed for the stabilization of anaerobically curable adhesives. On the basis of their mechanism they can essentially be subdivided into three groups:
1. Precursors for free radicals which are able to react with macroradicals of a growing chain so that as a result more stable free radicals are formed which prevent chain growth and consequently polymerization. This group includes standard anti-oxidants such as phenols, hydroquinones, sterically hindered phenols and hydroquinones, quinones, diphenylamine derivatives, etc. Nitrones (German published application No. 2,441,963) can also be placed in this group. The addition of hydroquinones to anaerobically curable adhesive mixtures increases their stability during storage but at the same time they loose the capability to harden on metal substrates. Sterically hindered phenols do not have this disadvantageous effect to the same extent but their stabilizing effect is not significant. The also known N-nitrosamines and hydroxyl amines have carcinogenic properties so that their use cannot be considered.
2. Stable free radicals which can react with macroradicals and stop the chain growth are effective as inhibitors. These include nitroxyls according to German Pat. No. 2,060,645. They have the disadvantage of being compounds with a clearly defined half-life period which therefore gradually decompose during storage. A further disadvantage of these compounds is that they delay the complete curing of anaerobic adhesives on non-ferrous metals, such as zinc.
3. Compounds which reduce the metal content of the monomers by compound or complex formation, which also increases the stability of adhesive mixtures. These compounds more particularly include chelating agents, such as derivatives of ethylene diamine, tetraacetic acid (EDTA), cf German published application No. 2,113,094. This stabilization method has the disadvantage that when the adhesives are used on metals, it is necessary to eliminate the action of the chelating agent before the hardening process starts which increases the required curing time.